Fraudulent usage of cellular and other telephone networks is a growing threat for both network subscribers and operators. Telecommunications providers worldwide lose billions of dollars to abusive and/or fraudulent usage of their networks each year. Fraud detection systems currently deployed typically use predefined rules with static thresholds to flag suspicious activity. Criminal individuals and organizations, however, employ increasingly sophisticated and ever-evolving techniques to thwart such defenses. In one type of fraudulent activity, referred to as PBX hacking, hackers penetrate the phone systems of firms and place calls to premium rate numbers. PBX hacking is estimated to cost the industry over $4 billion per year. In another fraud approach, known as Wangiri fraud, criminals use automated dialing services to ring and immediately hang up on thousands of users at once, inducing them to dial the missed number, an overseas premium rate line. Perpetrators executing this attack cost carriers approximately $2 billion globally each year. In another $5 billion scheme, subscription fraud, stolen credit card and identity information is used to set up new phone plans. Exorbitant usage during the initial billing period follows and the bill is never paid. Overall, carriers sacrifice an estimated 5% of their yearly revenue to fraudulent network usage.